The present invention relates to the art of casting, and more particularly to a method of making a mold for use in casting and having an accurately positioned core to form an accurately located passage in the finished product.
In the casting of metal it may be desirable to provide a hollow passage through the inside of the finished product. Such a passage may be formed by attaching a ceramic core to the inside of a mold. When molten metal is poured into the mold, the core excludes the metal from the areas occupied by the core. Once the metal has solidified, the mold and the core are removed leaving a casting having a passage of the desired shape.
Problems have been encountered when using this technique to form hollow passages. In some castings the location of the passage in the finished product is critical. Prior known techniques have had difficulty in precisely locating the cores with the result that the passages have not been accurately located. This has resulted in a large number of unsatisfactory castings.
Further difficulties have been encountered when firing a mold with a core attached inside. The prior practice has been to fire the ceramic core in an oven before attaching it to a mold. The fired cores are embedded in a wax pattern of the object to be cast. The wax pattern is then repeatedly dipped in a ceramic slurry to build a "green" coating of ceramic material on the wax pattern. The green mold is then fired in an oven. The firing melts out the wax and hardens the ceramic material into a mold strong enough to withstand the impact of molten metal.
One result of firing a ceramic material is a slight change in the coefficient of thermal expansion. The core which is attached to the inside of the green mold has already been fired. Thus the thermal expansion coefficient of the core differs somewhat from the thermal expansion coefficient of the unfired ceramic material which forms the still green mold. The difference in thermal expansion coefficients can cause either the core or the mold to crack when they are connected with each other and fired. Firing may also cause the core to shift positions.
The damage to the core and the mold occurs during firing. This damage may not be outwardly visable. The damage may only be detectable after the casting has been poured and the once molten metal has hardened. The damage then appears as a passage through the finished piece in the wrong position caused by a shift in the core position. The passage also may be incomplete because of a broken core. This hidden damage is expensive because it cannot be detected until the casting process is complete.